To improve competitiveness in the semiconductor industry, unit processes capable of improving the yield of a semiconductor device have been developed. In addition, studies related to methods and apparatus for measuring process errors in each unit process have been actively pursued. The photolithography process is typically a main semiconductor manufacturing process. During a photolithography process, conditions may be varied. Accordingly, manufacturers may develop a process for controlling the variation of the photolithographic process conditions and an apparatus for performing the variation control process.
A problem that may be considered when performing a photolithographic process is misalignment of a photoresist pattern that is formed through exposure and development processes. As a semiconductor device may be highly integrated, an alignment margin generally decreases and a wafer size increases, which may make a precise alignment very difficult. Thus, photoresist pattern misalignment may be problematic in semiconductor manufacturing.
A conventional photolithographic process will now be described. First, a photoresist film is spin-coated on a wafer. Then, a coordinate value of an alignment mark, which has been formed through a photolithographic process, is read out in an exposure device by checking an intensity variation of diffracted light caused by a laser having a monochromatic light beam, or by checking a difference in darkness created by a broadband white light beam. Thereafter, a compensating value for correcting the coordinate value is calculated so as to precisely align the wafer to a predetermined position.
FIG. 1 illustrates a conventional wafer aligning apparatus. A method for aligning the wafer by using the alignment mark will be now be described with reference to FIG. 1. The wafer W coated with a photoresist film is loaded on a wafer stage 10. In the wafer W, a scribe line area is provided between chips and the alignment marks and overlay marks are formed in the scribe line area. Light 12 is radiated onto the wafer in a direction vertical to the alignment marks formed in the wafer W. At this time, the light 12 is continuously radiated onto the alignment marks, which are provided in the form of plural diffraction gratings. The radiated light 12 is diffracted by the alignment marks and a primary light or a secondary light of the diffracted light 14 is sensed. The sensed diffracted light 14 is displayed as a DC signal waveform that is obtained by photoelectrically converting sensed diffracted light 14. A position coordinate of the alignment marks is obtained from the signal waveform. The position of the alignment marks formed at each area of the wafer W is also detected through the above procedures. Then, the position coordinate of the alignment mark, which is preset when designing the alignment marks, is compared with the position coordinate of the alignment mark obtained through the above procedures to align the wafer W to a relatively precise position.
After aligning the wafer W as discussed above, light, including ultraviolet ray, electronic-beam, or X-ray is radiated onto the photoresist film so as to selectively expose the photoresist film. Then, the photoresist film is patterned through a development process. Thereafter, it is determined whether or not the pattern formed in a present step is identical to the pattern formed in a former step by using an overlay key of an overlay measuring apparatus.
A measuring result of an overlay is then read out. If a measured overlay value is “spec-in,” a next procedure, such as an etching procedure, is carried out. If the measured overlay value is “spec-out,” a compensating value for correcting the misalignment is calculated and the exposure and development procedures are carried out again. If, however, the wafer W is aligned by radiating light 12 onto the alignment marks as described above, steps of the alignment marks may be evenly distributed over the whole area of the wafer to precisely align the wafer W. If the steps in each pattern of the alignment marks are different at different areas of the wafer W, then a measured position coordinate of the alignment marks of the wafer W is different from a real position coordinate of the alignment marks of the wafer W.
FIG. 2 illustrates sensing positions of the diffracted light according to the steps of the alignment marks in a conventional aligning apparatus. Referring now to FIG. 2, when the alignment marks 20 formed in the wafer W are positioned at the same coordinate, if the alignment marks 20 have different heights by an amount ΔZ, a sensing position of the diffracted light diffracted by an alignment mark 20a having a relatively high height is different from a sensing position of diffracted light diffracted by an alignment mark 20b having a relatively low height. In more detail, the sensing position of the diffracted light diffracted by an alignment mark 20b having a relatively low height is shifted in a positive (+) direction by an amount ΔX as compared with the sensing position of diffracted light diffracted by an alignment mark 20a having a relatively high height. That is, position errors may be incurred due to step differences of the alignment marks.
Step differences may be created when performing a chemical mechanical polishing (CMP) process, which is a common semiconductor manufacturing process. Because the CMP process planarizes an entire surface of the wafer, the alignment marks may be polished or damaged through the CMP process. In addition, if the entire surface of the wafer is unevenly polished, a height difference between patterns included in the alignment marks may become greater at each area of the wafer.
In addition, when a higher-order diffracted light is sensed from the diffracted light diffracted by the alignment marks, an angle between a vertically radiated light and the sensed higher-order diffracted light may increase. Accordingly, the position of the alignment marks measured by higher-order diffracted light may have a significant error. Thus, if the wafer is aligned according to the position coordinate of the alignment marks measured based on the sensing position of diffracted light, then the wafer may not be precisely aligned.
According to the procedures described above, a wafer is aligned by radiating light in a direction vertical to the alignment marks formed in the wafer. It is also possible, however, to align a wafer by slantingly radiating light onto the alignment marks. Japanese Laid-Open Patent Publication No. 4-237114 discloses a method and an apparatus for controlling light transmission by slantingly radiating an aligning light beam onto diffraction gratings of a mask and a wafer. In addition, Japanese Laid-open Patent Publication No.62-058628 discloses a method for aligning a position by radiating light beams having wavelengths of different frequencies.